It also works for units you acquire with a timer for how long until they'll die, like in a secret mission I found there were machines..kinda like goliaths, and they had timers for how long until they died. Once the timer was done, they were still there, it also worked for portable flame thrower turrets I used in an underground mission on char.

EDIT: Oh yeah, and in one mission against Protoss to get one of the artifact pieces there's these fields..rip fields I believe, and they emit a radiation or something that will even damage your invincible units but an unusual thing I noticed is that none of the units actually died, I just had some that were yellow or orange in color.

[Edited by Aeterna85, 7/30/2010 1:20:55 PM]

The HP of your units gets reset when they're hit. So just make sure your units get hit by some turrets or aircrafts in this mission and they wont die.